One exemption, one rate, nothing else
Straight from the statute: 32 V.S.A. §7442a(b) sets the entire rate schedule as two lines: "Under $5,000,000.00: None" and "$5,000,000.00 or more: 16 percent of the excess over $5,000,000.00". There's no ladder of brackets to climb through — every dollar above the exemption is taxed at the same flat rate.
A flat rate replaced an older, more complex system
Vermont didn't always work this way. Before 2016, Vermont computed its estate tax using an old federal credit-table method similar to what several other states still use today. Legislation that year (Act relating to Creation of a Flat Rate Vermont Estate Tax) replaced that entire mechanism with the simple flat-rate structure in effect now.
No portability, but an unlimited marital deduction
Vermont doesn't let a surviving spouse add a deceased spouse's unused exemption to their own. See Portability & Marital Deduction for how the marital deduction works instead.